NReco.Recommender free version (available on
github
) is published under AGPL license; it can be used for free in GPL projects and for educational/testing purposes.

Buying a commercial license is mandatory as soon as you develop commercial activities distributing the NReco Recommender
inside your product or deploying it on a network without disclosing the source code of your own applications under the AGPL license.
These activities include:

offering paid services to customers as an ASP

making recommendations in the cloud or in a web application

shipping NReco Recommender with a closed source product

documentation

Part 1 is about making recommendations: what is collaborative filtering, how to make recommendations with Mahout.
All code snippets are applicable to Recommender.
This intoduction is strongly recommended if you're new to collaborative filtering and recommendation algorithms.